Automatic transmissions have long employed hydraulically-applied clutches to effect ratio changes in planetary gear trains, for example, through the synchronized opening and closing of select clutches attached to different elements of the planetary systems. The clutch assembly used in automatic transmissions typically consists of multiple separators, friction plates and other components. The friction plate is made of a low carbon steel core plate with friction material bonded on each side, which is splined to the input shaft while the separator plates are lugged to the clutch housing or vice versa. The clutch housing could link to another shaft (rotating clutches) or to the transmission case (brake clutches).
Due to their desirable friction characteristics for shift quality and cost effectiveness, wet paper-based friction materials have been primarily used as clutch facings for automatic transmissions in cars and brake materials for industrial vehicles. However, because of the lack of heat and chemical stability under extremely high temperature conditions, the paper-based cellulose fiber often breaks down causing material break out, high-friction-lining thickness loss and, eventually, loss of appropriate friction characteristics. Indeed, it is believed that thermal degradation of the friction material due to high temperature is the major factor for determining the life cycle of a wet clutch.
After the friction material is thermally degraded to a certain extent, the friction characteristics will be changed dramatically and the performance of the friction material will suffer. Generally, the least heat-resistant composition in the friction material will be degraded first at elevated temperatures. By way of example only, in an exemplary wet friction clutch, a first rapid decline occurs at about 380° C. and corresponds to the rate of cellulose degradation. A second rapid decrease occurs at about 550° C. and corresponds to that of inert contents. For instance, some cellulose fibers start to carbonize near 200° C. and finish the degradation at about 420° C. under the Thermal Gravimetric Analysis (TGA) test condition.
The damage that occurs in a friction system from the temperature induced by high-energy shifts is cumulative. Each excursion to high temperatures activates the damage mechanism and adds damage to that already done. By reducing the peak interface temperatures and by reducing the time above the activation temperature, the damage to the friction material is reduced and the useful life of the friction system can be increased.
In addition to causing clutch failure, high clutch temperature also influences shift quality. It is well known that excellent shift quality depends on accurate clutch torque execution during a shift. Typically, clutch torque is a linear function of clutch friction coefficient (COF), which is a strong, nearly-linear function of temperature. Again, by way of example only, the COF of an exemplary clutch might vary 20-30% over a one-hundred degree temperature change. Thus, it will be appreciated that, without temperature compensation, shift quality is very difficult to maintain.